This invention relates to hydrophones and more particularly to an improved coaxial electret line hydrophone and method detecting underwater vibrations.
The coaxial electret cable described in U.S. Pat. No. 3,763,482 is an extemely sensitive transducer capable of detecting mechanical impacts and vibrations applied to the exterior of the cable at any point along its length. Such a transducer is useful as a hydrophone for sensing underwater vibrations such as soundings during undersea seismic exploration and the like. For example, the coaxial electret cable may be towed behind a surface vessel at depths up to 1,500 meters for sensing sonic vibrations reflected from the floor of the sea and providing output signals indicative of the character and composition of the sea floor. An embodiment of this cable in a hydrophone is described in U.S. Pat. No. 3,921,125.
A difficulty experienced with the use of the aforementioned electret cable as a hydrophone is a substantial loss of sensitivity when exposed to hydrostatic pressures equivalent to ocean depths of thousands of feet. Such loss of sensitivity is believed to be due to the increased stiffness and vanishing volume of the air trapped between the outer jacket or cover of the cable and the dielectric layer as hydrostatic pressure increases. As a result, a given acoustic signal applied to the cable exterior produces a greatly reduced radial displacement of the cable outer conductor (shield) relative to the electret (dielectric layer) at these higher static pressures. At lower static pressures, this trapped air is under less pressure and is therefore more compliant and voluminous so that radial displacement of the outer conductor relative to the electret layer is proportionally less restricted, resulting in a more sensitive transducer. The loss of cable transducer sensitivity with increasing hydrostatic pressures limits the usefulness of the cable as a hydrophone. This invention is directed to a solution to this problem.